Low Band Receiver; Receiver Front-End - Motorola 6864115B62-C Detailed Service Manual

2-2
2.0

Low Band Receiver

2.1

Receiver Front-End

The low band receiver is bandsplit into three ranges depending on radio model, covering frequencies
from 29.7 to 36.0 MHz, 36.0 to 42.0 MHz, or 42.0 to 50.0 MHz. The circuitry of the three models is
identical except for component value differences. The receiver consists of five major blocks: front-end
bandpass filters and pre-amplifier, first mixer, high-IF and blanker switches, low-IF and receiver back-
end, and "Extender" (noise blanker). Two fixed-tuned bandpass filters perform antenna signal pre-
selection. A cross over quad diode mixer converts the signal to the high-IF of 10.7 MHz. High-side
first injection is used.
Enable
Antenna
Pin Diode
Antenna
Switch
RF Jack
Recovered Audio
RSSI
Blanking Pulses
Noise
Blanker
Fixed
Tuned Filter
Demodulator
IF
455kHz Filter
(25/20kHz)
455kHz Filter
(12.5kHz)
Figure 2-1 Low Band Receiver Block Diagram
Delay
IF Amp
Filter
Fixed
RF Amp
Tuned Filter
First LO
RXINJ
Limiter
455kHz Filter
(25/20kHz)
455kHz Filter
(12.5kHz)
THEORY OF OPERATION
Buffer
Gate
1st
Mixer
4-pole
Crystal
IF Amp
Second LO
BWSELECT
Filter Bank Selection
from Synthesizer IC
(U1201 Pin 48)
Filter